The invention relates to a power grid filter choke as defined by the independent claim. Such chokes are used in the operation of such power electronics devices as switched-mode power supplies, servo drives, and UPSs.
A power grid filter limits interfering factors that are caused by electronic devices and that affect the public power supply grid (radio interference suppression). A power grid filter also improves the electromagnetic compatibility of electrical devices and thus increases the immunity to interference. There is accordingly a need for power grid filters with chokes in accordance with the invention, in order to meet the relevant electromagnetic compatibility specifications.
A common feature of these power grid filters is that they have capacitances in the range of a few microrfarads (phase measured against protective conductor), which are meant to keep asymmetrical interference away from the phase voltage of the power grid. These capacitances, together with the grid impedance, form a more or less damped oscillating circuit, which can come into resonance in the presence of suitable excitation.
The consequence is high leakage currents at the filter input as well as considerable phase-to-ground voltages with a corresponding frequency, which can be superimposed on the normal grid voltage and lead to the destruction of the components. The sources of these excitations are extremely various.
They can be the power electronics connected to the power grid filter, the pulse frequencies of which electronics and the associated harmonics typically extend up to several tens of a kilohertz, or even harmonics that are a multiple of the grid frequency and can be caused by completely different consumers that are connected to some other point of the grid. The spectrum of these excitations thus extends from a few hundred hertz as far as several tens of a kilohertz.
The impedance of a power supply grid is composed of a complex system, comprising infeed nodes, lines, transformers, and so forth, which essentially assure an inductive component, but also comprise the entirety of the consumers that are connected at a particular time. If ohmic consumers predominate, then the grid is well damped, and the risk of resonances is less. Each additional load reduces the effective grid impedance. Inductive components of the impedance are determined essentially by the stray reactances of transformers and by the line lengths (distance from infeed points), but also by inductive consumers, such as rotary-current motors. In very extensive grids, as for instance in North America and China, the inductive components are as a rule higher than in Europe. Hence the impedance of a grid cannot be predicted. It depends on the switching state and the particular load situation and therefore also varies over time.
Taking what has been said above into account, it becomes clear that there is a relatively high risk of resonances which nevertheless is difficult to predict. Even if the grid is stable and relatively well damped, resonances can occur if a power electronics consumer together with a capacitive load is operated on a relatively long, low-loss supply line, or in other words one that has a sufficiently large cross section. If a large power electronics system with many power grid filters is involved, such as a drive system for a printing press, then 20-30 m of lead line are already sufficient to shift the resonant frequency into a critical range.
Dictated by the pulsed operation that is established in power electronics, the parasitic capacitances to ground that are present in every system (for Instance conductor to shield In shield of motor lines), leakage currents occur, which to a large extent flow back via the capacitors present in the power grid filters (phase-ground) and thus represent the greatest proportion of the excitations. This is accordingly an asymmetrical excitation. In principle, a symmetrical excitation (that is, phase-phase) is also conceivable, but the likelihood of resonance in that case is substantially less, since the consumers downstream of the power grid filter typically provide for adequate symmetrical damping.
In order to provide for an asymmetrical damping, some power grid filter manufacturers connect low-impedance resistors parallel to the capacitors, but even in rated operation this leads to a corresponding power loss. Nor is it assured that the damping in the presence of relatively great excitation will still suffice to suppress resonances. Inserting resistors into the lead line is precluded, because of the enormous power loss that would cause.
German Published Utility Model DE 295 06 951 U1 shows the construction of a power grid filter with a damping transformer in the form of a choke for power grid filters, with a primary winding for connecting the choke to the grid phases L1 through L3, and with a secondary winding with a secondary circuit; the primary winding and the secondary winding are located on a winding core, and the secondary circuit includes a damping member in the form of an ohmic load that by means of the choke effects damping in the primary circuit. The ohmic load is transformed into the primary circuit by the transformational action of the arrangement and damps the primary circuit permanently.
The disadvantage of this arrangement is on the one hand this permanent damping of the primary circuit, and on the other the use of ferrite as the material for the coil core of the choke. Ferrite has the disadvantage that the permeability is highly temperature-dependent, and the saturation induction amounts to only approximately 400 mT. The Curie temperatures are also so low that they can easily be reached when used for a damping choke, where high losses are intentionally meant to occur as needed.
Ferrite ring cores can furthermore be manufactured economically only up to a certain size, since the danger of breakage during production rises sharply with increasing size, leading to a low yield and correspondingly high prices. Moreover, a possible overload cannot be detected. With the choke known from the prior art, only the cable-based interference caused by the power electronics is meant to be reduced, specifically because the resonance points are shifted to a lower frequency range that is not relevant for the electromagnetic compatibility consideration.